Detecting the presence of specific chemical compounds in the atmosphere is important in a variety of different applications. For example, it is often important to detect the presence and concentration of potentially flammable compounds in the atmosphere. Chemical compounds of interest are often referred to as target analytes.
A variety of different sensor systems known in the art can be used to detect the presence and concentration of different analytes. For example, conductiometric sensor systems, optical sensor systems, and surface acoustic wave sensor systems can all be used.
One type of conductiometric sensor is a polymer-absorption chemiresistor sensor. Polymer-absorption chemiresistor sensors include a sensor probe having a pair of electrodes and a sensing element. The probe is part of a sensor circuit.
The sensing element typically takes the form of a polymeric sensor film that spans the two electrodes. The sensor film is exposed to the surrounding atmosphere. The exact composition of the polymeric sensor film varies depending on the target analyte, as is known in the art, such that the sensor film absorbs the target analyte when it is present in the surrounding atmosphere.
A load is applied across the sensor film via the electrodes. Upon exposure to and absorption of the target analyte, the sensor film swells and undergoes a volumetric change. The change in volume changes the electrical resistance of the film.
A processor or control unit is typically coupled to the sensor circuit. The processor monitors the resistance of the sensor film to determine the absence, presence, and concentration of the target analytes. The processor can be coupled to a user interface. The user interface typically includes an indicating device that generates a signal when the concentration of the target analyte exceeds a predetermined threshold value.
The resistance of the sensor film changes not only in response to absorption of the target analytes, but also in response to changes in ambient temperature. If the sensor film has a positive temperature coefficient of resistance, the resistance of the sensor film increases as ambient temperature increases. If the sensor film has a negative temperature coefficient of resistance, the resistance of the sensor film decreases as ambient temperature increases. Whether the sensor film has a positive or negative temperature coefficient of resistance depends on the composition of the sensor film and the application.
Because detection of target analytes is based on changes in the resistance of the sensor film that occur when the sensor absorbs target analytes, changes in ambient temperature that change the resistance of the sensor film can negatively affect the sensor system's ability to accurately detect the presence of target analytes. For example, if the sensor film has a positive temperature coefficient of resistance and increases in resistance upon the absorption of target analytes, increases in ambient temperature might cause the sensor system to generate a false signal indicating that target analytes are present when they are not.
While conventional chemiresistor sensor systems perform adequately for their intended uses, they are subject to improvement. Specifically, there is a need for a chemiresistor sensor system that can modify its overall resistance in response to changes in ambient temperature to increase the accuracy of the sensor system.